Variable resistance control with differentially resilient contacts

ABSTRACT

A variable resistance control having a contactor with differentially resilient contacts wipably engaging a resistance element and a collector for eliminating intermittency and decreasing dynamic contact resistance.

United States Patent 91 Robinson et al.

1 51 Dec. 17, 1974 1 VARIABLE RESISTANCE CONTROL WITH DIFFERENTIALLYRESILIENT CONTACTS [75] Inventors: James II. Robinson; John D. VanBenthuysen, both of Elkhart, Ind.

[73] Assignee: CTS Corporation, Elkhart, Ind.

[22] Filed: Apr. 2, 1974 [21] Appl. No.: 457,182

[52] us. Cl 338/171, 338/174, 338/202 [51] Int. Cl I-IOlc 9/02 [58]Field of Search 338/171, 202, 167, 169, 338/174,175,127

[56] References Cited UNITEDv STATES PATENTS 2,093,252 9/1937Schellenger 338/174 2,177,291 10/1939 Schellenger..... 338/202 X2,632,830 3/1953 Aust et al. 3,576,514 4/1971 Michik 338/l7l X PrimaryExaminer-Bruce A. Reynolds Attorney, Agent, or Firm.lohn J. Gaydos 57ABSTRACT A variable resistance control having a contactor withdifferentially resilient contacts wipably engaging a resistance elementand a collector for eliminating intermittency and decreasing dynamiccontact resistance.

9 Claims, 6 Drawing Figures frequently in variable resistance controlsas resistivity increases and the size of the control decreases isintermittency. Higher resistivities and smaller controls are being usedin ever increasing numbers as the demand forelectronic equipmentcontaining solid state devices increases. Currently there is a large andincreasing demand for small preset variable resistance controls fortrimming circuits in solid state television and stereo sets hence thecontrols are generally referred to as preset trimmers.

Trimming a circuit requires the introduction of a resistance into thecircuit until an optimum condition occurs. For example, if 42,000 i 100ohms of a 100,000

to obtain within limits the precise resistance normallyobtainable fromthe control. Very likely the same control could be connected in anothervirtually identical circuit and satisfactorily perform its intendedfunction when a slightly higher or lower resistance is required becausethe identical electrical circuit parameters almost never are duplicatedin two circuits using the same components. Obviously, the advantage ofusing a preset control to trim a circuit is decreased if it is necessaryto replace the control before the circuit can be properly trimmed. Itwould therefore be desirable to manufacture a variable resistancecontrol that can be adjusted frorh one end of the resistance range tothe other end'thereof without becoming intermittent.

Various types of tests are performed on variable resistance controls tomonitor and/or graphically identify the characteristics thereof. Forexample, electrical noise occurs in the circuit while the contactor isbeing wiped across the resistance element and is normally identified asequivalent noise resistance" (ENR). The

ENR or dynamic contact resistance is measurable in ohms and as a percentof the total resistance by following standarized test procedures wellknown in the art. Again with the advent of solid state devices such astransistors and integrated circuits, which are extremely quiet incomparison with vacuum tubes, it has become increasingly important toreduce the ENR of variable resistance controls. Various theories arecontinually being propounded for decreasing the ENR of a variableresistance control. The use of double contacts as exemplified in U.S.Pat. No. 2,178,283 for wipingly engaging an arcuate resistance elementcan be employed to decrease the ENR of a variable resistance control.According to the prior art, the contacts should be designed to exert thesame amount of pressure on the arcuate resistance element to ensuresmoother and more dependable operation of the control. Such equalizationof pressures exertable by a pair of contacts generally is satisfactoryfor controls with low resistivities, but more problems occur whencontrols with higher resistivities are used with-solid state devices. Itwould,therefore, be desirable to obtain variable resistance controlsthroughout the entire resitivity range with decreased ENR.

Accordingly, it is an object of the present invention to provide a newand improved variable resistance control having various desirablefeatures as those disclosed above. Another object of the presentinvention is to provide a variable resistance control with a contactorhaving a pair of differentially resilient contacts wipably engaging theresistance element of a variable resistance control. A further object ofthe present invention is to provide a variable resistance control with acontactor having arms with different spring rates for supporting thecontacts. Another object of the present invention is to provide avariable resistance control with an ENR lower than currently obtainable.Further objects and advantages of the present invention will becomeapparent as the following description proceeds, and the features ofnovelty characterizing the invention will be pointed out withparticularity in the claims annexed to and forming a part of thisspecification.

Briefly, the present invention is concerned with a variable resistancecontrol having a contactor with. differentially resilient contactswipably engaging a resistance element and a collector for eliminatingintermittency anddecreasing dynamic contact resistance The contactorcomprises a metal ring, a pair of outer arms integral with the metalring and extending outwardly therefrom toward each other for supportingthe differ" entially resilient outer contact wipably engaging theresistance element, an inner arm integral with the metal ring extendingoutwardly toward the outer contact and supporting the differentiallyresilient inner contact wipably engaging the resistance element, and amain contact on the ring wipably engaging the collector. The metal ringcomprises a pair of semicircular sections performed to define an obtuseangle therebetween.

For a better understanding of the present invention, reference may behad to the accompanying drawings wherein the same reference numeralshave been applied to like parts and wherein:

FIG. 1 is an isometric view of an improved variable resistance controlbuilt in accord with the present invention;

FIG. 2 is an enlarged sectional view taken along lines IIII of FIG. 1;

FIG. 3 is a graph depicting curves of the resistance gradient and ENR asa percent of the total resistance;

bracket 11, a base 20, a rotatable member 30 and an 1 equalizingcontactor 40. As best seen in FIG. 2 of the drawing, the supportingbracket 11 comprises a metal stamping having a snap-in center terminal12 extending downwardly from the center portion of the bracket formounting the control to a panel or the like. A collector ring 13embossed from the center portion of the bracket extends inwardly thereofinto an opening 21 of the base for aligning the base with the bracket11. Tabs 14 (see FIG. 1) projecting forwardly of the bracket 11 engagenotchess 22 in the base for preventing relative rotation therebetween.An arcuate carbon resistance element 23 is secured to the base with apair of end terminals 24. The contactor 40 constrained to rotate withthe member wipably engages the collector ring l3 and the resistanceelement intermediate the ends thereof for providing an infinite numberof resistance values between one of the end terminals 24 and the centerterminal 12 electrically connected to the collector ring 13. For a morethorough description of the control shown in FIG. 1, reference should bemade to U.S. Pat. No. 3,375,478 incorporated herein by reference.

Preferably, and in accord with the present invention, the contactor 40made from metal comprises a body portion or circular ring 41 (see FIG.5) having a pair ofpivots 42 extending outwardly from the circular ringfor .pivotally supporting the contactor against the rotatable member 30thereby permitting pivotal action of the contactor with respect to theresistance element 23 and the collector ring 13. Each of the pivots 42are provided with an upturned end 42a further constraining the contactorto move with the rotatable member 30.

The circular ring 41 of the contactor 40 is folded about a line 41apassing through the pivots 42. Such folding or performing divides thecircular ring in half and forms a pair of semicircular sections 43, 44having an obtuse angle 41b therebetween. A pair of outer arms 45integral with and extending tangentially outwardly from opposite ends ofthe semi-circular section 43 converge towrd each other and areintegrally joined togeter (see FIG. 5) at their extremities 45a. In apreferred form of the invention, an inner arm 46 extends radiallyoutwardly from the semicircular section 43 of the contactor 40 towardthe junction or outer extremity 45a of the outer arms 45. Although anouter contact 47 and an inner contact 48 are formed directly from theouter and inner arms, respectively, it is to be understood that thecontacts 47, 48 can be fixedly secured to the extremities of the outerandinner arms. As best shown in FIG. 5 of the drawings, the outer arms45 define an acute angle 45b and the inner arm bisects the acute angle45b with the outer contact 47 being spaced from the inner contact 48.Moreover, in accord with the present invention, the length of each ofthe outer arms 45 is greater than the length of the inner arm 46resulting in a different spring rate for the outer arm and the innerarm. The difference in spring rates of the arms 45, 46 carryingrespectively the inner and outer contacts 47, 48 produces a pair ofdifferentially resilient contacts 47, 48 wipably engaging the resistanceelement 23'. Further, the difference in spring rate between the innerand outer amrs 45, 46 is increased since the outer arms 45 flex as leafsprings throughout their length while the inner arm 46, being shorterand stubbier,.flexes generally as a torsion spring because of thetwisting affect of the portions 43a, 43b of the semicircular section 43between the inner arm and the outer arms. According to the presentinvention, the inner contact 48 can be carried by more than one arm solong as the effective pres-- sure exerted by the inner contact 48against the resistance element 23 is different than the pressure exertedby the outer contact 47. A main contact 49 carried by the semicircularsection 44 wipably engages the collector ring 13 and electricallyconnects the inner and outer contacts 47, 48 wipably engaging theresistance element 23 to the center terminal 12.

A large number of controls as exemplified and shown in FIG. 1 of thedrawings are employed for trimming electronic circuits. The controls areadjusted to make the equipment operable and then usually are notadjusted unless certain components are replaced in the circuit. Suchcontrols are designated as factory adjust controls while other controlsadjusted periodically by the user are commonly referred to as useradjust controls. When the control 10 is employed for trimming electroniccircuits, e.g., one of the circuits in a television set, it isimperative that a particular resistance value be obtainable from thecontrol otherwise the circuit cannot be properly trimmed. A control isintermittent when a particular resistance value cannot be obtainedbecause the contactor 40 as best shown in FIG. 4 is electricallyinsulated from the resistance element, i.e., the contact 47 rests on anonconductive portion of the resistance element, assuming that contact48 is not a part of the contactor 40. A thorough study indicates thatthe nonconductive portion 23a of the resistance element 23 is usually aclump ofnonconductive binder or a small particle produced during theshearing operation of the laminated fiber forming the substrate 23b ofthe resistance element and bonded to the substrate with the binder. Suchlaminated fiber particles 23a are frequently electrostatically attractedto the substrate and are very difficult to remove prior to applicationof the carbon resistance paint onto the substrate.

The resistance obtainable between one of the end terminals 24 of thecontrol 10 and the center terminal 12 as the contactor is moved from oneend of the resistance element to the other is graphically depicted inFIG. 3 of the drawings, and is commonly referred to as a resistancegradient wave 50. The resistance gradient curve 50 of a control depictsthe change in resistance obtained by measuring the resistance betweenthe contact wipingly engaging the resistance element and one of the endterminalsas a function of rotation or movement of the contactor from oneend of the resistance element to the other end thereof. An idealresistance gradient curve would be depicted as a straight line having aslope determined by the total resistance as a function of totalrotation. A careful study of intermittent controls, that is, controlshaving an excessively high or infinite resistance at some degree ofrotation between the center terminal and one of the end terminalsreveals that such controls produce a spike 51 in the resistance gradientcurve such as shown in FIG. 3.

When trimming a circuit, if the optimum or desired resistance cannot beobtained because the single contact of a control rests on an insulatedparticle, the control would be intermittent. But, in accord with thepresent invention, when such condition exists, the inner contact 48 isstill wipingly engaging the resistance element permitting furtheradjustment of the control to obtain the optimum resistance for trimmingthe circuit. The likelihood of having both the inner and outer contactssimultaneously resting on insulated particles is rather remote. It is,therefore, apparent that the control is no longer intermittent since thespike 51 will disappear on the resistance gradient curve 50.

When the control is designed in a circuit as a user adjust control,it-is necessary that the contact resistance be kept to a minimum.Contact resistance is of little importance when the control 10 is usedas a trimmer since any resistance in or between the contact and theresistance element of the control becomes a part of the total resistancein the circuit. However, in certain applications where the control isfrequently adjusted by the user, it is preferable that the contactresistance commonly referred to as equivalent noise resistance (ENR) bekept to a minimum and/or as uniform as possible. As shown in FIG. 3, theENR curve 52 of a control with a single contact is greater than the ENRcurve 53 of a double contact such as when the inner and outer contactsof the presentinvention wipably engage the resistance element. ENR isessentially a measurement of the contact resistance as the contactwipingly moves across the resistance element. During such dynamicconditions, the contact resistance becomes substantial, e.g., theENRcu'rve can become as high as 10 percent. Generally an ENR curve for asingle contact control is considered satisfactory if below threepercent. The average ENR curve for controls employing the contactor 40is less than 1% percent and slightly higher for prior art double paddlecontactors. An increase in contact resistance from a static to a dynamiccondition results from the movement and bouncing of the contacts acrossthe resistance element. Since the pair of arms 45 functioning as springssupporting the outer contact'47 have a different spring rate than thearm 46 supporting the inner contact 48, and since the outer arms 45function generally as leaf springs while the inner arm remains morerigid and generally functions as a torsion spring resulting fromtwisting of the semicircular sections extending from opposite sides ofthe inner arm, the spring rate of the inner contact 48 will be differentfrom the spring rate of the outer contact 47. This difference in springrate between the two springs effectively alters the bounce or frequencyof thetwo contacts wipingly engaging the resistance element andapparently results in a slightly lower ENR than would otherwise beobtained. Further, the current density or gradient through a mid sectionof the resistance element 23 from the inner edge to the outer edgethereof generally follows an exponential curve such as shown in FIG. 6of the drawings, the high current density being at the inner edge of theelement 23. It has been found that by making the pressure of the innercontact 48 at least several percent greater than the 'pressure of theouter contact 47 against the element a slightly improved ENR curve isobtainable.

While there has been illustrated and described what is at presentconsidered to be a preferred embodiment of the present invention, itwill be appreciated that numerous changes and modifications are likelyto occur to those skilled in the art, and it is intended in theapsecured by means for moving the contactor intermediate the ends of theresistance element, said contactor comprising a circular ring, a pair ofpivots extendingoutwardly from the body portion and pivotallysupportingthe contactor against the resistance element and the collector, each ofthe pivots being provided with an upturned end constraining thecontactor to move with the above mentioned means, the circular bodyportion being creased through its pivots and defining a pair ofsemicircular sections, each of the semicircular sections of the circularbody portion being preformed and defining an obtuse angle therebetween,a pair of outer arms extending tangentially outwardly from one of thesemicircular sections and converging toward each other and joinedtogether at the extremities, an outer contact carried by the arms, aninner arm extending radially outwardly from the one of the semicircularsections toward the outer contact, an inner contact carried by the endof the inner arm, and spaced from outer contact, the outer contact andthe inner contact wipably engaging the resistance element, the outerarms defining an acute angle, and the inner arm bisecting the acuteangle, the length of the outer arms being greater than the length of theinner arm resulting in a different rate for the inner armand the outerarms, and a main contact carried by the circular body portion andwipably engaging the collector.

2. The variable resistance control of claim 1, wherein the inner contactexerts a greater pressure against the resistance element than the outercontact.

3. In a variable resistance control having a resistance element, acollector in spaced relationship to the resistance element,anelectrically conductive contactor engaging the resistance element andthe collector, means for moving the contactor intermediate the ends; ofthe resistance element, said contactor comprising a body portion, a pairof outer arms extending outwardly from opposite sides of the bodyportion and converging toward each other, an outer contact carried bythe arms and wipably engaging the resistance element, an inner armintegral with the body portion and extending outwardly therefrom towardthe outer contact, the end of said inner arm being spaced from the outercontact, an inner contact disposed on the distal end of the inner arm inspaced relationship to the outer contact and wipably engaging theresistance element, and a main contact carried by the body portion andwipably engaging the collector.

4. The variable resistance control of claim 3, wherein the body portionof the contactor is a metal ring defined by a pair of semicircularsections, each of the semicircular sections of the ring being preformedand defining an obtuse angle therebetween.

S. The variable resistance control of claim 4, wherein the outer andinner arms are integral with one of the semicircular sections of themetal ring, and the main contact is integral with the other of thesemicircular sections of the metal ring.

6. The variable resistance control of claim 3, wherein the outer armsdefine an acute angle and the innerarm bisects the acute angle.

7. The variable resistance control of claim 3, wherein the length ofeach of the outer arms is greater than the length of the inner armresulting in a different spring rate for the inner arm and the outerarms.

8. The variable resistance control of claim 4, wherein the outer armsextend tangentially outwardly from the one of the semicircular sectionsand the inner arm extends radially outwardly from the one of thesemicircular sections.

9. The variable resistance control of claim 3, wherein the inner contactexerts a greater pressure against the resistance element than the outercontact.

1. A variable resistance control comprising a resistance element, acollector in spaced relationship to the resistance element, anelectrically conductive contactor engaging the resistance element andthe collector, means for moving the contactor intermediate the ends ofthe resistance element, said contactor comprising a circular ring, apair of pivots extending outwardly from the body portion and pivotallysupporting the contactor against the resistance element and thecollector, each of the pivots being provided with an upturned endconstraining the contactor to move with the above mentioned means, thecircular body portion being creased through its pivots and defining apair of semicircular sections, each of the semicircular sections of thecircular body portion being preformed and defining an obtuse angletherebetween, a pair of outer arms extending tangentially outwardly fromone of the semicircular sections and converging toward each other andjoined together at the extremities, an outer contact carried by thearms, an inner arm extending radially outwardly from the one of thesemicircular sections toward the outer contact, an inner contact carriedby the end of the inner arm, and spaced from outer contact, the outercontact and the inner contact wipably engaging the resistance element,the outer arms defining an acute angle, and the inner arm bisecting theacute angle, the length of the outer arms being greater than the lengthof the inner arm resulting in a different rate for the inner arm and theouter arms, and a main contact carried by the circular body portion andwipably engaging the collector.
 2. The variable resistance control ofclaim 1, wherein the iNner contact exerts a greater pressure against theresistance element than the outer contact.
 3. In a variable resistancecontrol having a resistance element, a collector in spaced relationshipto the resistance element, an electrically conductive contactor engagingthe resistance element and the collector, means for moving the contactorintermediate the ends of the resistance element, said contactorcomprising a body portion, a pair of outer arms extending outwardly fromopposite sides of the body portion and converging toward each other, anouter contact carried by the arms and wipably engaging the resistanceelement, an inner arm integral with the body portion and extendingoutwardly therefrom toward the outer contact, the end of said inner armbeing spaced from the outer contact, an inner contact disposed on thedistal end of the inner arm in spaced relationship to the outer contactand wipably engaging the resistance element, and a main contact carriedby the body portion and wipably engaging the collector.
 4. The variableresistance control of claim 3, wherein the body portion of the contactoris a metal ring defined by a pair of semicircular sections, each of thesemicircular sections of the ring being preformed and defining an obtuseangle therebetween.
 5. The variable resistance control of claim 4,wherein the outer and inner arms are integral with one of thesemicircular sections of the metal ring, and the main contact isintegral with the other of the semicircular sections of the metal ring.6. The variable resistance control of claim 3, wherein the outer armsdefine an acute angle and the inner arm bisects the acute angle.
 7. Thevariable resistance control of claim 3, wherein the length of each ofthe outer arms is greater than the length of the inner arm resulting ina different spring rate for the inner arm and the outer arms.
 8. Thevariable resistance control of claim 4, wherein the outer arms extendtangentially outwardly from the one of the semicircular sections and theinner arm extends radially outwardly from the one of the semicircularsections.
 9. The variable resistance control of claim 3, wherein theinner contact exerts a greater pressure against the resistance elementthan the outer contact.